"=" operator vs. "IS"

Hi.

I'm just curious - why is it not possible to use the "=" operator to
compare values with NULL? I suspect that the SQL standard specified
it that way, but I can't see any ambiguity in an expression like "AND
foo.bar = NULL". Is it because NULL does not "equal" any value, and the
expression should be read as "foo.bar is unknown"? Or is there something
else I'm missing?

If it's the "unknown" part, then why can't I use "=" to compare with
TRUE or FALSE?


cheers,
stefan